//  register for our events
Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginRequest);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endRequest);

function beginRequest(sender, args) {
    // show the popup
    $find('mdlProgress').show();
}

function endRequest(sender, args) {
    //  hide the popup
    $find('mdlProgress').hide();
}